Unify web-player design tokens and componentize collection headers

Migrates remaining one-off colors (bg-white/[N%], border-white/N, hardcoded
shadows/gradients) to the existing --mf-* tokens so light mode reacts
consistently everywhere, including a fix to --color-hover which was
hardcoded instead of pointing at --mf-surface-2. Extracts CollectionHeader
(album/playlist/mix/liked headers) and AccountMenu (TopBar/MobileHeader)
to remove five and two duplicated implementations respectively, and
normalizes inconsistent empty-state copy.

web-player/src/lib/components/ui/SectionHeading.svelte

@@ -12,7 +12,7 @@
 </script>
 
 {#snippet heading()}
-	<h2 class="font-display text-[17.5px] font-medium tracking-[-0.025em] text-fg">
+	<h2 class="font-display text-lg font-medium tracking-[-0.025em] text-fg">
 		{title}
 	</h2>
 	{#if subtitle}